Composite membranes are well known.
For example, U.S. Pat. Nos. 4,711,719 and 4,562,021 describe methods of manufacturing membranes with separating layers made of Al.sub.2 O.sub.3. This involves preparing a sol containing boehmite (AlOOH) particles of size lying in the range of 3 nm to 2 .mu.m and which are in suspension in a liquid phase comprising water, an acid such as HNO.sub.3 or HClO.sub.4 and optionally a thickening agent such as polyvinyl alcohol. This suspension is then put into contact with the support to deposit a layer which is subsequently dried and heated until the boehmite is transformed into alumina and until the alumina particles are sintered into a layer constituting a porous body of alumina which is strongly bonded to the support.
Throughout the following, the thermal stability of these layers is discussed. Thermal stability is a property of porous structures in which crystal or grain size, specific surface area and pore size evolve to a small extent only when these structures are heated to high temperatures.
When the above-defined layers are used at high temperatures, their pore size is not sufficiently stable. This is due to the fact that porous bodies are inherently metastable: any mobility of atoms or groups of atoms tends to lower surface energy by lowering specific surface area.
In practice, this tendency causes grain growth which increases pore size and sometimes decreases pore fraction per unit volume. This effect is mentioned in U.S. Pat. Nos. 4,711,719 and 4,562,021 which describe how the pore size of the final layer can be increased by simply increasing its sintering temperature. If the mobility of atoms or groups of atoms at a given temperature is sufficient to cause grain growth and the associated increase in pore diameter to occur during the sintering operation which generally only lasts a few hours, then the pore size stability of the layer cannot be maintained during practical use of the membrane over periods of several thousand hours unless the temperature of use is considerably lower than the sintering temperature. This means that the temperature of use is limited to too low a value for many applications.
The problem of insufficient thermal stability occurs mainly in membranes having pores that are very small in diameter, i.e. diameters smaller than 50 nm. For example, an alumina membrane sintered at 1,400.degree. C., with a pore diameter of 1 .mu.m, may be maintained at 800.degree. C. for a very long time without any noticeable change in the diameter of its pores. For membranes with small pores, i.e. with small grains of oxide, the main source of mobility of the atoms is free surface diffusion and grain boundary diffusion.
A similar problem is encountered in catalysts or catalyst supports made of gamma-alumina and having a large specific surface area. When these catalysts are used for long periods at high temperature, their specific surface area decreases, their grain diameter increases, and the gamma-alumina is converted to alpha-alumina. This decreases the surface area available for catalysis or for lodging a finely dispersed catalyst. The efficiency of the catalyst is thus reduced.
Proposals have already been made to impregnate such gamma-alumina catalyst supports with a solution of a salt of an oxide other than Al.sub.2 O.sub.3. This impregnation is followed by drying and calcination. The end result is the formation of another oxide in the pores of the catalyst support or on their surfaces. It is observed that the thermal stability of the catalyst is increased when zirconium, calcium, lanthanum, or thorium salts are used, but is decreased when indium or gallium salts are used.
If such a process is applied to the separating layer of a membrane, the following drawbacks are observed:
the impregnation brings the impregnation oxide onto the free surface of the grains forming the layer, but does not bring it into the grain boundaries, and therefore has no effect on atom mobility in grain boundaries; and PA1 in addition to the impregnation proper, the impregnation process involves a drying operation and a baking operation, and is therefore costly. PA1 said seperating layer is composed of grains of a first oxide phase and grains of at least one second oxide which is dispersed over and chemically bonded to the first oxide phase, so as to constitute at least a portion of the free surface of the grains of said first oxide phase and at least a portion of the grain boundaries between the grains of said first oxide phase; PA1 said first oxide phase and said second oxide are chosen so that the second oxide slows down diffusion both at the surface and at the grain boundary of the first oxide phase and has low solubility in said first oxide phase; and PA1 the volume of said second oxide lies in the range of 0.1% to 25% of the volume of said first oxide phase. PA1 preparing a stable sol of particles of said first oxide phase, or of the corresponding hydroxide; PA1 adding to this sol at least one compound of at least one metal capable of forming said second oxide, said compound being capable of being mixed homogeneously with the liquid phase of said sol to obtain a slip; PA1 putting the microporous inorganic support into contact with said slip to deposit said separating layer; and PA1 drying this layer and heating it to a temperature such that said hydroxide is converted to oxide (if hydroxide is being used), and so that said first oxide phase and said second oxide are sintered together to form a microporous structure in which said second oxide is dispersed both on the free surface of the grains of the first oxide phase and in the grain boundaries thereof. PA1 the compound dissolves in said liquid phase and remains in a dissolved state; or PA1 the compound dissolves and then precipitates onto the surface of the collodal particles of the sol; or PA1 the compound reacts with one of the constituents of the sol, the reaction product then precipitating onto the surface of the colloidal particles of the sol.
An object of the present invention is to provide a porous inorganic composite semipermeable membrane which is crack-free, and stable at high temperatures.